This invention relates to devices for collecting a plurality of control data from a programmable controller memory by means of a data collecting unit connected to the programmable controller through a bus, outputting these collected data to a monitor device such as an image display, and causing these control data to be displayed sequentially in time and monitored by the monitor device.
A programmable controller (PLC) is formed by using a bus to connect a CPU unit for a control, a communication unit for communicating with the exterior and an I/O unit for connecting to sensors. The CPU unit contains a microprocessor and a user memory that stores user programs preliminarily created according to the details of the control. The I/O refresh process, the user program execution process and the peripheral service process are cyclically carried out by executing a specified system program. The time required for carrying out this cycle of processes is generally referred to as a cycle time. The I/O refresh process is for the CPU unit of the PLC to exchange control data such as IN data and OUT data through the bus communication with the I/O unit. The CPU unit includes memories (such as an input-output memory and a data memory) for storing control data such as IN data and OUT data. Data collecting units connectable to a PLC have been known, as disclosed in Japanese Patent Publication Tokkai 2005-128721, as an example of device for automatically and periodically collecting control data of all sorts stored in the memory of the CPU unit (inclusive of the aforementioned IN data and OUT data and more specifically the on-off data of input-output signals). Such a data collecting unit continues to collect data from the CPU unit at a specified frequency and stores a time-series of changes of control data. The data collecting unit is sometimes herein referred to as a signal processing unit (SPU) and is connected to the CPU unit through a bus. It has also been known, as disclosed in Japanese Patent Publication Tokkai 2003-21650, to monitor through a monitor device such as an image display device a series of control data collected by such a data collecting unit. As a series of such control data is stored, the on-off signals of an input-output signal varying as a time series (a time series of bit data array) can be grasped by a monitor device to be described below.
It is common for a monitor device of this kind to be provided with the function of receiving a series of control data collected from the CPU unit by the data collecting unit, converting each time series of changes of these data into a pulse array waveform, and displaying them over a plurality of lines on the display screen.
In order to be collected by a data collecting unit of this type, the data must be treated as control data or variable by the PLC. Moreover, they must be assigned to the I/O memory or the data memory contained in the CPU unit, and this is to say that the recording area of the memory must be uniquely determined. When it is desired to collect special data such as those obtained by a calculation between two variables by means of the data collecting unit, for example, it is necessary to set new variables and to correct the user program (such as a program created in a ladder diagram language) such that the PLC will carry out a calculation between the variables. The data collecting unit must also be set such that these news variables will be periodically collected from the assigned areas of the memory of the CPU unit of the PLC and that the monitor device will read out these data and display.
For example, although variables such as the number of produced objects and the number of defective objects are usually defined within the data memory of a PLC, if it is desired to separately obtain data such as the number of OK objects and the ratio of OK objects, variables must be newly defined corresponding to them and calculation formulas corresponding to these variables must be added by using a specified instruction language in the user program of the PLC. Specifically, the calculation formulas that may be written as “Number of OK objects”=“Number of produced objects”−“Number of defective objects” and “Ratio of OK objects”=(“Number of produced objects”−“Number of defective objects”)/“Number of produced objects” must be added into the user program.
One generally hesitates to add or correct a user program after the PLC has already entered its operating cycle and is normally operating, although it may become desirable to monitor new data involving a calculation between variables. This is because the newly added calculation may cause an increase in the cycle time and even an unexpected operational error. As a result, it is common that the manner of using the monitoring device becomes limited after the PLC enters its operating stage.
It is therefore an object of this invention in view of the above to provide a monitor system capable of monitoring new data involving calculations between variables even after the PLC has entered an operating stage without adding or modifying the user program of the PLC.
Other objects of this invention will become clear from the descriptions of the specification that follows.